Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antenna elements are connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose, adding together (interfering constructively) to enhance the power radiated in desired directions, and cancelling (interfering destructively) to reduce the power radiated in other directions.
FIG. 1 is a diagram illustrating the basic principle of MIMO communications, for a system with two transmitters and two receivers. A transmitter 101 has two antennas 102, 103, which transmit signals to a receiver 104, which has two antennas 105, 106. The signals propagate through different paths 107, 108, 109, 110. This feature may be used to provide improved performance by exploiting spatial diversity to improve reliability in poor channel conditions or to increase the data rate by spatial multiplexing in good conditions.
The Long term Evolution (LTE) standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. LTE release 13 supports spatial multiplexing for 16 transmit (Tx) antennas with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. Typically such systems involve a technique known as precoding, which involves the application of phase and gain shifts to optimise multipath propagation. FIG. 2 is a schematic diagram illustrating the application of precoding to signals. FIG. 2 illustrates a plurality of data streams or layers 201. The data streams are represented by an information carrying vector s 202, which comprises a plurality of symbols r. Each of the r symbols corresponds to a layer. r is referred to as the transmission rank. The symbol vector s 202 is multiplied 203 by an NT×r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. An Inverse Fast Fourier 204 Transform applied. The signals are then sent to antenna ports 205. The ports may then be mapped onto antenna elements. This mapping may involve the simple mapping of one port to one antenna, or it may involve the mapping of the signals to combinations of antenna elements by means of a virtualization matrix.
In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same Time/Frequency Resource Element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (and Discrete Fourier Transform (DFT) precoded OFDM in the uplink) and hence the received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modelled by:yn=HnWsn+en  (1)where en is a noise/interference vector obtained as realizations of a random process. Hn is the channel matrix, which represents the effects of a multipath channel on a signal between a given transmitter and a given receiver. Hn is an NR×NT matrix, wherein NT is the number of transmitters and NR is the number of receivers.
The precoder matrix can be a wideband precoder, which is constant over frequency, or frequency selective. The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. The aim is to focus the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. Furthermore, the transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder. For efficient performance it is therefore of critical importance to obtain information about the channel Hn, this information is commonly referred to as channel state information (CSI). A technique for determining the channel state information is to provide a reference signal, which provides a known symbol, which, on detection, can be used to determine the channel state. This type of signal is known as a Channel State Information reference signal (CSI-RS).
There are two main methodologies for transmitting CSI-RS in a system with many steerable antennas: non-precoded or precoded CSI-RS. With non-precoded CSI-RS, a single CSI-RS resource is used, which comprises many antenna ports, and typically a separate CSI-RS port is transmitted from each (possibly virtual) antenna element of the array, so that the UE can estimate the full high-dimensional channel matrix from the many antenna ports. Typically, the UE would then feed-back a CSI report indicating a high-dimensional precoder, such as a DFT precoder. The CSI-RS in this case is then intended to be cell-specific, i.e. broadcasted over the entire cell coverage area. FIG. 3 is a schematic diagram illustrating a non-precoded CSI-RS. There is a multi-antenna array 301, which transmits to user equipment 302. The non-precoded CSI-RS 303 is transmitted by the base station across the cell.
The UE then provides feedback 304 in the form of a Rank Indicator (RI), a Precoder Matrix Index, PMI, and a Channel Quality Indicator (CQI). The RI corresponds to a recommended number of streams that are to be spatially multiplexed and thus transmitted in parallel over the effective channel. The PMI identifies a recommended precoder (in a codebook which contains precoders with the same number of rows as the number of CSI-RS ports) for the transmission, which relates to the spatial characteristics of the effective channel. The CQI represents a recommended transport block size (i.e. code rate) and LTE supports transmission of one or two simultaneous (on different layers) transmissions of transport blocks (i.e. separately encoded blocks of information) to a UE in a subframe. There is thus a relation between a CQI and an SINR of the spatial stream(s) over which the transport block or blocks are transmitted.
Precoded CSI-RS on the other hand can be either UE-specific or cell-specific. In the cell-specific case, typically many CSI-RS resources are transmitted, each resource typically comprising only one antenna port per polarization. The CSI-RS within a CSI-RS resource is typically transmitted from all antenna elements of the array, but precoded with certain beamforming weights to create a narrow beam in a certain direction. The UE would then measure upon all CSI-RS resources and select the best one, corresponding to the best beam direction, and feed back a CSI-RS Resource Indicator (CRI). FIGS. 3 and 4 are diagrams illustrating precoded CSI-RS. FIG. 4 is a schematic diagram of a single precoded CSI RS, which illustrates an antenna array 401 transmitting a single reference signal 403, which has been precoded to form a beam, to a UE 402. FIG. 5 is a schematic diagram which illustrates an antenna array 501 transmitting multiple precoded CSI-RS 503, 504, 505, 506 to at least one UE 502.
Using LTE terminology, non-precoded CSI-RS transmission schemes are denoted “Class A eMIMO-Type” while beamformed CSI-RS transmission schemes are denoted “Class B eMIMO-Type”.
A common type of precoding is to use a DFT-precoder, where the precoder vector used to precode a single-layer transmission using a single-polarized uniform linear array (ULA) with N1 antennas is defined as:
                                          w                          1              ⁢                                                          ⁢              D                                ⁡                      (                          l              ,                              N                1                            ,                              O                1                                      )                          =                              1                                          N                1                                              ⁡                      [                                                                                e                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                              π                        ·                        0                                            ⁢                                              l                                                                              O                            1                                                    ⁢                                                      N                            1                                                                                                                                                                                                                e                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                              π                        ·                        1                                            ⁢                                              l                                                                              O                            1                                                    ⁢                                                      N                            1                                                                                                                                                                                            ⋮                                                                                                  e                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                              π                        ·                                                  (                                                                                    N                              1                                                        -                            1                                                    )                                                                    ⁢                                              l                                                                              O                            1                                                    ⁢                                                      N                            1                                                                                                                                                                    ]                                              (        2        )            
wherein l=0, 1, . . . O1N1−1 is the precoder index and O1 is an integer oversampling factor. A precoder for a dual-polarized uniform linear array (ULA) with N1 antennas per polarization (and so 2N1 antennas in total) can be similarly defined as
                                          w                                          1                ⁢                                                                  ⁢                D                            ,              DP                                ⁡                      (                          l              ,                              N                1                            ,                              O                1                                      )                          =                              [                                                                                                      w                                              1                        ⁢                                                                                                  ⁢                        D                                                              ⁡                                          (                      l                      )                                                                                                                                                              e                                              j                        ⁢                                                                                                  ⁢                        ϕ                                                              ⁢                                                                  w                                                  1                          ⁢                                                                                                          ⁢                          D                                                                    ⁡                                              (                        l                        )                                                                                                                  ]                    =                                    [                                                                                                                  w                                                  1                          ⁢                                                                                                          ⁢                          D                                                                    ⁡                                              (                        l                        )                                                                                                  0                                                                                        0                                                                                                      w                                                  1                          ⁢                                                                                                          ⁢                          D                                                                    ⁡                                              (                        l                        )                                                                                                        ]                        ⁡                          [                                                                    1                                                                                                              e                                              j                        ⁢                                                                                                  ⁢                        ϕ                                                                                                        ]                                                          (        3        )            
where ejϕ is a co-phasing factor between the two polarizations that may for instance be selected from a QPSK alphabet
  ϕ  ∈            {              0        ,                  π          2                ,        π        ,                              3            ⁢            π                    2                    }        .  
Recent development in 3GPP has led to the discussion of two-dimensional antenna arrays where each antenna element has an independent phase and amplitude control, thereby enabling beamforming in both in the vertical and the horizontal dimension. Such antenna arrays may be (partly) described by the number of antenna columns corresponding to the horizontal dimension Mh, the number of antenna rows corresponding to the vertical dimension Mv and the number of dimensions corresponding to different polarizations Mp. The total number of antennas is thus M=MhMvMp. A special subset of 2D antenna arrays are 1D arrays which is the set of antenna arrays where Mh=1 and Mv>1 or Mh>1 and Mv=1. FIG. 6 is a representation of such an antenna design. It comprises an array of m by n cross-polarized antenna elements 601. In this example Mh=4, Mv=8 I and Mp=2. Such an antenna is denoted as an 8×4 antenna array with cross-polarized antenna elements.
The concept of an antenna element is non-limiting in the sense that it can refer to any virtualization (e.g., linear mapping) of a transmitted signal to the physical antenna elements. For example, groups of physical antenna elements could be fed the same signal, and hence they share the same virtualized antenna port when observed at the receiver. Hence, the receiver cannot distinguish and measure the channel from each individual antenna element within the group of element that are virtualized together. Therefore, when transmitting for instance CSI-RS corresponding to NT antenna ports it is not necessarily so that NT equals the number of antenna elements used for the transmission. Hence, the number of antenna elements and the number of antenna ports may or may not need equal each other.
A corresponding precoder vector for a two-dimensional uniform planar arrays (UPA) with N1×N2 antennas can be created by taking the Kronecker product of two precoder vectors as:w2D(l,m)=w1D(l,N1,O1)⊗w1D(m,N2,O2)  (4)
where O2 is an integer oversampling factor in the N2 dimension. Each precoder w2D(l,m) forms a DFT beam, all the precoders {w2D(l,m), l=0, . . . N1O1−1; m=0, . . . , N2O2−1} form a grid of DFT beams. FIG. 7 is a representation of such a set of precoder beams, in which (N1,N2)=(4,2) and (O1, O2)=(4,4). FIG. 7 illustrates a plurality of orthogonal DFT beams 701 and oversampled beams 702. A specific example of a DFT precoder corresponding to w2D(l=2, m=1) is given 703. The terms DFT beams' and DFT precoders' are used interchangeably.
Extending the precoder for a dual-polarized ULA may then be done as
                                          w                                          2                ⁢                                                                  ⁢                D                            ,              DP                                ⁡                      (                          l              ,              m              ,              ϕ                        )                          =                                            [                                                                    1                                                                                                              e                                              j                        ⁢                                                                                                  ⁢                        ϕ                                                                                                        ]                        ⊗                                          w                                  2                  ⁢                                                                          ⁢                  D                                            ⁡                              (                                  l                  ,                  m                                )                                              =                                                                 [                                                                                                                              w                                                      2                            ⁢                                                                                                                  ⁢                            D                                                                          ⁡                                                  (                                                      l                            ,                            m                                                    )                                                                                                                                                                                                  e                                                      j                            ⁢                                                                                                                  ⁢                            ϕ                                                                          ⁢                                                                              w                                                          2                              ⁢                                                                                                                          ⁢                              D                                                                                ⁡                                                      (                                                          l                              ,                              m                                                        )                                                                                                                                              ]                            =                                                [                                                                                                                                          w                                                          2                              ⁢                                                                                                                          ⁢                              D                                                                                ⁡                                                      (                                                          l                              ,                              m                                                        )                                                                                                                      0                                                                                                            0                                                                                                                          w                                                          2                              ⁢                                                                                                                          ⁢                              D                                                                                ⁡                                                      (                                                          l                              ,                              m                                                        )                                                                                                                                ]                                ⁡                                  [                                                                                    1                                                                                                                                      e                                                      j                            ⁢                                                                                                                  ⁢                            ϕ                                                                                                                                ]                                                                                        (        5        )            
A precoder matrix W2D,DR for multi-layer transmission may be created by appending columns of DFT precoder vectors asW2D,DP(R)=[w2D,DP(l1,m1,ϕ1) w2D,DP(l2,m2,ϕ2) . . . w2D,DP(lR,mR,ϕR)]  (6)
where R is the number of transmission layers, i.e. the transmission rank. In a special case for a rank-2 DFT precoder, m1=m2=m and l1=l2=l:
                                          w                                          2                ⁢                                                                  ⁢                D                            ,              DP                                      (              2              )                                ⁡                      (                          l              ,              m              ,                              ϕ                1                            ,                              ϕ                2                                      )                          =                              [                                                            w                                                            2                      ⁢                                                                                          ⁢                      D                                        ,                    DP                                                  ⁡                                  (                                      l                    ,                    m                    ,                                          ϕ                      1                                                        )                                            ⁢                                                          ⁢                                                w                                                            2                      ⁢                                                                                          ⁢                      D                                        ,                    DP                                                  ⁡                                  (                                      l                    ,                    m                    ,                                          ϕ                      2                                                        )                                                      ]                    =                                                                 [                                                                                                                              w                                                      2                            ⁢                                                                                                                  ⁢                            D                                                                          ⁡                                                  (                                                      l                            ,                            m                                                    )                                                                                                            0                                                                                                  0                                                                                                                w                                                      2                            ⁢                                                                                                                  ⁢                            D                                                                          ⁡                                                  (                                                      l                            ,                            m                                                    )                                                                                                                    ]                            ⁡                              [                                                                            1                                                              1                                                                                                                          e                                                  j                          ⁢                                                                                                          ⁢                                                      ϕ                            1                                                                                                                                                              e                                                  j                          ⁢                                                                                                          ⁢                                                      ϕ                            2                                                                                                                                              ]                                                                        (        7        )            
For each rank, all the precoder candidates form a ‘precoder codebook’ or a ‘codebook’. A UE can first determine the rank of the estimated downlink wideband channel based CSI-RS. After the rank is identified, for each sub-band the UE then searches through all the precoder candidates in a codebook for the determined rank to find the best precoder for the sub-band. For example, in case of rank=1, the UE would search through w2D,DP(k,l,ϕ) for all the possible (k,l,ϕ) values. In case of rank=2, the UE would search through W2D,DP(2)(k,l,ϕ1,ϕ2) for all the possible (k,l,ϕ1,ϕ2) values.
With non-precoded CSI-RS, the UE can estimate the full-dimensional channel and feed-back a precoder selection. Thus, the resulting Physical Downlink Shared Channel (PDSCH) beam depends on precoder codebook, which can be very large and have a high spatial granularity.
With DFT codebooks, this corresponds to using a high oversampling factor. However, each CSI-RS is only transmitted using a single antenna element, which has two downsides:                Inefficient power amplifier (PA) utilization: Each CSI-RS is transmitted from only a single PA, which means that only a fraction of the total TX power can be used. This power loss can be mitigated by boosting the power on the CSI-RS by “borrowing” TX power from empty REs where CSI-RS from other antenna ports are transmitted. However, this can only be done to some extent without causing PA linearization problems and intolerable out-of-band emissions. The power loss can also be mitigated by having an OCC across several CSI-RS ports, but this requires that the channel is sufficiently static in time and frequency.        Poor coverage: As the CSI-RS ports are transmitted from a single antenna element, it will not experience any beamforming gain, and so the coverage may be poor, especially for high frequencies where the path loss is significant.        
With precoded CSI-RS, on the other hand, each CSI-RS is transmitted from the entire array and so has full PA utilization and the coverage is good since a beamforming gain is experienced. However, precoded CSI-RS suffers from poor spatial resolution compared to the non-precoded CSI-RS scheme as the UE can only select between the beams transmitted on the different CSI-RS resources. Thus, the spatial granularity depends on the number of transmitted CSI-RS and not on the codebook size, so increasing the spatial granularity requires additional DL overhead. If the precoded CSI-RS scheme uses the same number of ports as the non-precoded CSI-RS scheme, the spatial granularity corresponds to a DFT oversampling factor of one, whereas the non-precoded scheme can use any oversampling factor, typically four or eight per spatial dimension, leading to 16-64 times larger spatial granularity.